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INTRODUCTION

Ticks are blood-feeding external parasites that pose a significant infectious disease risk to humans and animals worldwide (Figure 123-1). They have been implicated as vectors in the transmission of more than 200 pathogens (e.g., babesiosis, ehrlichiosis, Lyme disease, Rocky Mountain spotted fever, tick-borne relapsing fever, tick paralysis, and tularemia).1-3 Disease transmission occurs when stomach contents and saliva from the tick are introduced into the host during the blood-feeding process. Transmission of infectious agents by ticks is closely related to the duration of tick attachment and blood-feeding. Lyme disease is the most common tick-borne illness in the United States.4 The risk of contracting it increases significantly once a tick has remained attached for more than 24 to 36 hours.5,6 This is the time required for bacteria to migrate from the midgut of a tick to the salivary glands.5-7 The prevention of disease transmission relies on early and effective removal of attached ticks.8-11

ANATOMY AND PATHOPHYSIOLOGY

Ticks are arachnids and divided into Argasidae (i.e., soft ticks) and Ixodidae (i.e., hard ticks).12 Hard body ticks are responsible for the transmission of most human diseases and will be the focus of this chapter. Hard ticks pass through four life cycle stages from birth (i.e., egg, larva, nymph, and adult). They require a blood meal to progress to the next stage of their development.

Ticks are often encountered in the late spring, summer, and early fall. Ticks are more prevalent in rural and wooded areas. They like to feed on dark (i.e., covered) and moist areas of the body (e.g., axilla, groin, and scalp). The bite of a tick is painless, often goes unnoticed, and the tick is found attached to the skin.

Ixodes ticks have a shield-like scutum on the dorsum of their body with mouthparts that protrude forward. Nymphs and adults have eight legs. They vary in size depending on life cycle stage. Nymphal ticks are approximately 1.5 mm long. Adult unfed ticks are about 3 mm long, although once fed they can enlarge to 11 mm in length.13 Tick coloration varies based on species and stage of feeding. Unfed ticks often appear black or brown. Engorged ticks during or after feeding can appear pink or dark red.14 Ticks have specialized mouthparts that make their removal difficult (Figure 123-2).12 They screw their mouthparts into the skin in a clockwise direction. Mouthparts include the palps, the chelicerae, and the hypostome. Ticks cut through skin using the chelicerae. Ticks then insert their hypostome, a tube-like structure through which the feeding takes place.13 The hypostome has many backward-facing sharp barbs called denticles that prevent the tick from being dislodged. The tick can secrete a ...

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